tools/sched_ext: add arena based scheduler

Add a scheduler that uses BPF arenas to manage task context data.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/sched_ext/Makefile

@@ -189,7 +189,7 @@ $(INCLUDE_DIR)/%.bpf.skel.h: $(SCXOBJ_DIR)/%.bpf.o $(INCLUDE_DIR)/vmlinux.h $(BP
 
 SCX_COMMON_DEPS := include/scx/common.h include/scx/user_exit_info.h | $(BINDIR)
 
-c-sched-targets = scx_simple scx_cpu0 scx_qmap scx_central scx_flatcg scx_userland scx_pair
+c-sched-targets = scx_simple scx_cpu0 scx_qmap scx_central scx_flatcg scx_userland scx_pair scx_sdt
 
 $(addprefix $(BINDIR)/,$(c-sched-targets)): \
 	$(BINDIR)/%: \

tools/sched_ext/scx_sdt.bpf.c

@@ -0,0 +1,710 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Arena-based task data scheduler. This is a variation of scx_simple
+ * that uses a combined allocator and indexing structure to organize
+ * task data. Task context allocation is done when a task enters the
+ * scheduler, while freeing is done when it exits. Task contexts are
+ * retrieved from task-local storage, pointing to the allocated memory.
+ *
+ * The main purpose of this scheduler is to demostrate arena memory
+ * management.
+ *
+ * Copyright (c) 2024-2025 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024-2025 Emil Tsalapatis <etsal@meta.com>
+ * Copyright (c) 2024-2025 Tejun Heo <tj@kernel.org>
+ *
+ */
+#include <scx/common.bpf.h>
+#include <scx/bpf_arena_common.bpf.h>
+
+#include "scx_sdt.h"
+
+char _license[] SEC("license") = "GPL";
+
+UEI_DEFINE(uei);
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARENA);
+	__uint(map_flags, BPF_F_MMAPABLE);
+#if defined(__TARGET_ARCH_arm64) || defined(__aarch64__)
+	__uint(max_entries, 1 << 16); /* number of pages */
+        __ulong(map_extra, (1ull << 32)); /* start of mmap() region */
+#else
+	__uint(max_entries, 1 << 20); /* number of pages */
+        __ulong(map_extra, (1ull << 44)); /* start of mmap() region */
+#endif
+} arena __weak SEC(".maps");
+
+#define SHARED_DSQ 0
+
+#define DEFINE_SDT_STAT(metric)				\
+static inline void				\
+stat_inc_##metric(struct scx_stats __arena *stats)	\
+{							\
+	cast_kern(stats);				\
+	stats->metric += 1;				\
+}							\
+__u64 stat_##metric;					\
+
+DEFINE_SDT_STAT(enqueue);
+DEFINE_SDT_STAT(init);
+DEFINE_SDT_STAT(exit);
+DEFINE_SDT_STAT(select_idle_cpu);
+DEFINE_SDT_STAT(select_busy_cpu);
+
+/*
+ * Necessary for cond_break/can_loop's semantics. According to kernel commit
+ * 011832b, the loop counter variable must be seen as imprecise and bounded
+ * by the verifier. Initializing it from a constant (e.g., i = 0;), then,
+ * makes it precise and prevents may_goto from helping with converging the
+ * loop. For these loops we must initialize the loop counter from a variable
+ * whose value the verifier cannot reason about when checking the program, so
+ * that the loop counter's value is imprecise.
+ */
+static __u64 zero = 0;
+
+/*
+ * XXX Hack to get the verifier to find the arena for sdt_exit_task.
+ * As of 6.12-rc5, The verifier associates arenas with programs by
+ * checking LD.IMM instruction operands for an arena and populating
+ * the program state with the first instance it finds. This requires
+ * accessing our global arena variable, but scx methods do not necessarily
+ * do so while still using pointers from that arena. Insert a bpf_printk
+ * statement that triggers at most once to generate an LD.IMM instruction
+ * to access the arena and help the verifier.
+ */
+static volatile bool scx_arena_verify_once;
+
+__hidden void scx_arena_subprog_init(void)
+{
+	if (scx_arena_verify_once)
+		return;
+
+	bpf_printk("%s: arena pointer %p", __func__, &arena);
+	scx_arena_verify_once = true;
+}
+
+
+private(LOCK) struct bpf_spin_lock alloc_lock;
+private(POOL_LOCK) struct bpf_spin_lock alloc_pool_lock;
+
+/* allocation pools */
+struct sdt_pool desc_pool;
+struct sdt_pool chunk_pool;
+
+/* Protected by alloc_lock. */
+struct scx_alloc_stats alloc_stats;
+
+
+/* Allocate element from the pool. Must be called with a then pool lock held. */
+static
+void __arena *scx_alloc_from_pool(struct sdt_pool *pool)
+{
+	__u64 elem_size, max_elems;
+	void __arena *slab;
+	void __arena *ptr;
+
+	elem_size = pool->elem_size;
+	max_elems = pool->max_elems;
+
+	/* If the chunk is spent, get a new one. */
+	if (pool->idx >= max_elems) {
+		slab = bpf_arena_alloc_pages(&arena, NULL,
+			div_round_up(max_elems * elem_size, PAGE_SIZE), NUMA_NO_NODE, 0);
+		if (!slab)
+			return NULL;
+
+		pool->slab = slab;
+		pool->idx = 0;
+	}
+
+	ptr = (void __arena *)((__u64) pool->slab + elem_size * pool->idx);
+	pool->idx += 1;
+
+	return ptr;
+}
+
+/* Alloc desc and associated chunk. Called with the allocator spinlock held. */
+static sdt_desc_t *scx_alloc_chunk(void)
+{
+	struct sdt_chunk __arena *chunk;
+	sdt_desc_t *desc;
+	sdt_desc_t *out;
+
+	chunk = scx_alloc_from_pool(&chunk_pool);
+	if (!chunk)
+		return NULL;
+
+	desc = scx_alloc_from_pool(&desc_pool);
+	if (!desc) {
+		/*
+		 * Effectively frees the previous chunk allocation.
+		 * Index cannot be 0, so decrementing is always
+		 * valid.
+		 */
+		chunk_pool.idx -= 1;
+		return NULL;
+	}
+
+	out = desc;
+
+	desc->nr_free = SDT_TASK_ENTS_PER_CHUNK;
+	desc->chunk = chunk;
+
+	alloc_stats.chunk_allocs += 1;
+
+	return out;
+}
+
+static int pool_set_size(struct sdt_pool *pool, __u64 data_size, __u64 nr_pages)
+{
+	if (unlikely(data_size % 8))
+		return -EINVAL;
+
+	if (unlikely(nr_pages == 0))
+		return -EINVAL;
+
+	pool->elem_size = data_size;
+	pool->max_elems = (PAGE_SIZE * nr_pages) / pool->elem_size;
+	/* Populate the pool slab on the first allocation. */
+	pool->idx = pool->max_elems;
+
+	return 0;
+}
+
+/* Initialize both the base pool allocators and the root chunk of the index. */
+__hidden int
+scx_alloc_init(struct scx_allocator *alloc, __u64 data_size)
+{
+	size_t min_chunk_size;
+	int ret;
+
+	_Static_assert(sizeof(struct sdt_chunk) <= PAGE_SIZE,
+		"chunk size must fit into a page");
+
+	ret = pool_set_size(&chunk_pool, sizeof(struct sdt_chunk), 1);
+	if (ret != 0)
+		return ret;
+
+	ret = pool_set_size(&desc_pool, sizeof(struct sdt_desc), 1);
+	if (ret != 0)
+		return ret;
+
+	/* Wrap data into a descriptor and word align. */
+	data_size += sizeof(struct sdt_data);
+	data_size = round_up(data_size, 8);
+
+	/*
+	 * Ensure we allocate large enough chunks from the arena to avoid excessive
+	 * internal fragmentation when turning chunks it into structs.
+	 */
+	min_chunk_size = div_round_up(SDT_TASK_MIN_ELEM_PER_ALLOC * data_size, PAGE_SIZE);
+	ret = pool_set_size(&alloc->pool, data_size, min_chunk_size);
+	if (ret != 0)
+		return ret;
+
+	bpf_spin_lock(&alloc_lock);
+	alloc->root = scx_alloc_chunk();
+	bpf_spin_unlock(&alloc_lock);
+	if (!alloc->root)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static
+int set_idx_state(sdt_desc_t *desc, __u64 pos, bool state)
+{
+	__u64 __arena *allocated = desc->allocated;
+	__u64 bit;
+
+	if (unlikely(pos >= SDT_TASK_ENTS_PER_CHUNK))
+		return -EINVAL;
+
+	bit = (__u64)1 << (pos % 64);
+
+	if (state)
+		allocated[pos / 64] |= bit;
+	else
+		allocated[pos / 64] &= ~bit;
+
+	return 0;
+}
+
+static __noinline
+int mark_nodes_avail(sdt_desc_t *lv_desc[SDT_TASK_LEVELS], __u64 lv_pos[SDT_TASK_LEVELS])
+{
+	sdt_desc_t *desc;
+	__u64 u, level;
+	int ret;
+
+	for (u = zero; u < SDT_TASK_LEVELS && can_loop; u++) {
+		level = SDT_TASK_LEVELS - 1 - u;
+
+		/* Only propagate upwards if we are the parent's only free chunk. */
+		desc = lv_desc[level];
+
+		ret = set_idx_state(desc, lv_pos[level], false);
+		if (unlikely(ret != 0))
+			return ret;
+
+		desc->nr_free += 1;
+		if (desc->nr_free > 1)
+			return 0;
+	}
+
+	return 0;
+}
+
+/*
+ * Free the allocated struct with the given index. Called with the
+ * allocator lock taken.
+ */
+__hidden
+int scx_alloc_free_idx(struct scx_allocator *alloc, __u64 idx)
+{
+	const __u64 mask = (1 << SDT_TASK_ENTS_PER_PAGE_SHIFT) - 1;
+	sdt_desc_t *lv_desc[SDT_TASK_LEVELS];
+	sdt_desc_t * __arena *desc_children;
+	struct sdt_chunk __arena *chunk;
+	sdt_desc_t *desc;
+	struct sdt_data __arena *data;
+	__u64 level, shift, pos;
+	__u64 lv_pos[SDT_TASK_LEVELS];
+	int ret;
+	int i;
+
+	if (!alloc)
+		return 0;
+
+	desc = alloc->root;
+	if (unlikely(!desc))
+		return -EINVAL;
+
+	/* To appease the verifier. */
+	for (level = zero; level < SDT_TASK_LEVELS && can_loop; level++) {
+		lv_desc[level] = NULL;
+		lv_pos[level] = 0;
+	}
+
+	/* Find the leaf node containing the index. */
+	for (level = zero; level < SDT_TASK_LEVELS && can_loop; level++) {
+		shift = (SDT_TASK_LEVELS - 1 - level) * SDT_TASK_ENTS_PER_PAGE_SHIFT;
+		pos = (idx >> shift) & mask;
+
+		lv_desc[level] = desc;
+		lv_pos[level] = pos;
+
+		if (level == SDT_TASK_LEVELS - 1)
+			break;
+
+		chunk = desc->chunk;
+
+		desc_children = (sdt_desc_t * __arena *)chunk->descs;
+		desc = desc_children[pos];
+
+		if (unlikely(!desc))
+			return -EINVAL;
+	}
+
+	chunk = desc->chunk;
+
+	pos = idx & mask;
+	data = chunk->data[pos];
+	if (likely(data)) {
+		data[pos] = (struct sdt_data) {
+			.tid.genn = data->tid.genn + 1,
+		};
+
+		/* Zero out one word at a time. */
+		for (i = zero; i < alloc->pool.elem_size / 8 && can_loop; i++) {
+			data->payload[i] = 0;
+		}
+	}
+
+	ret = mark_nodes_avail(lv_desc, lv_pos);
+	if (unlikely(ret != 0))
+		return ret;
+
+	alloc_stats.active_allocs -= 1;
+	alloc_stats.free_ops += 1;
+
+	return 0;
+}
+
+static inline
+int ffs(__u64 word)
+{
+	unsigned int num = 0;
+
+	if ((word & 0xffffffff) == 0) {
+		num += 32;
+		word >>= 32;
+	}
+
+	if ((word & 0xffff) == 0) {
+		num += 16;
+		word >>= 16;
+	}
+
+	if ((word & 0xff) == 0) {
+		num += 8;
+		word >>= 8;
+	}
+
+	if ((word & 0xf) == 0) {
+		num += 4;
+		word >>= 4;
+	}
+
+	if ((word & 0x3) == 0) {
+		num += 2;
+		word >>= 2;
+	}
+
+	if ((word & 0x1) == 0) {
+		num += 1;
+		word >>= 1;
+	}
+
+	return num;
+}
+
+
+/* find the first empty slot */
+__hidden
+__u64 chunk_find_empty(sdt_desc_t __arg_arena *desc)
+{
+	__u64 freeslots;
+	__u64 i;
+
+	for (i = 0; i < SDT_TASK_CHUNK_BITMAP_U64S; i++) {
+		freeslots = ~desc->allocated[i];
+		if (freeslots == (__u64)0)
+			continue;
+
+		return (i * 64) + ffs(freeslots);
+	}
+
+	return SDT_TASK_ENTS_PER_CHUNK;
+}
+
+/*
+ * Find and return an available idx on the allocator.
+ * Called with the task spinlock held.
+ */
+static sdt_desc_t * desc_find_empty(sdt_desc_t *desc, __u64 *idxp)
+{
+	sdt_desc_t *lv_desc[SDT_TASK_LEVELS];
+	sdt_desc_t * __arena *desc_children;
+	struct sdt_chunk __arena *chunk;
+	sdt_desc_t *tmp;
+	__u64 lv_pos[SDT_TASK_LEVELS];
+	__u64 u, pos, level;
+	__u64 idx = 0;
+	int ret;
+
+	for (level = zero; level < SDT_TASK_LEVELS && can_loop; level++) {
+		pos = chunk_find_empty(desc);
+
+		/* If we error out, something has gone very wrong. */
+		if (unlikely(pos > SDT_TASK_ENTS_PER_CHUNK))
+			return NULL;
+
+		if (pos == SDT_TASK_ENTS_PER_CHUNK)
+			return NULL;
+
+		idx <<= SDT_TASK_ENTS_PER_PAGE_SHIFT;
+		idx |= pos;
+
+		/* Log the levels to complete allocation. */
+		lv_desc[level] = desc;
+		lv_pos[level] = pos;
+
+		/* The rest of the loop is for internal node traversal. */
+		if (level == SDT_TASK_LEVELS - 1)
+			break;
+
+		/* Allocate an internal node if necessary. */
+		chunk = desc->chunk;
+		desc_children = (sdt_desc_t * __arena *)chunk->descs;
+
+		desc = desc_children[pos];
+		if (!desc) {
+			desc = scx_alloc_chunk();
+			if (!desc)
+				return NULL;
+
+			desc_children[pos] = desc;
+		}
+	}
+
+	/*
+	 * Finding the descriptor along with any internal node
+	 * allocations was successful. Update all levels with
+	 * the new allocation.
+	 */
+	bpf_for(u, 0, SDT_TASK_LEVELS) {
+		level = SDT_TASK_LEVELS - 1 - u;
+		tmp = lv_desc[level];
+
+		ret = set_idx_state(tmp, lv_pos[level], true);
+		if (ret != 0)
+			break;
+
+		tmp->nr_free -= 1;
+		if (tmp->nr_free > 0)
+			break;
+	}
+
+	*idxp = idx;
+
+	return desc;
+}
+
+__hidden
+void __arena *scx_alloc(struct scx_allocator *alloc)
+{
+	struct sdt_data __arena *data = NULL;
+	struct sdt_chunk __arena *chunk;
+	sdt_desc_t *desc;
+	__u64 idx, pos;
+
+	if (!alloc)
+		return NULL;
+
+	bpf_spin_lock(&alloc_lock);
+
+	/* We unlock if we encounter an error in the function. */
+	desc = desc_find_empty(alloc->root, &idx);
+	if (unlikely(desc == NULL)) {
+		bpf_spin_unlock(&alloc_lock);
+		return NULL;
+	}
+
+	chunk = desc->chunk;
+
+	/* Populate the leaf node if necessary. */
+	pos = idx & (SDT_TASK_ENTS_PER_CHUNK - 1);
+	data = chunk->data[pos];
+	if (!data) {
+		data = scx_alloc_from_pool(&alloc->pool);
+		if (!data) {
+			scx_alloc_free_idx(alloc, idx);
+			bpf_spin_unlock(&alloc_lock);
+			return NULL;
+		}
+	}
+
+	chunk->data[pos] = data;
+
+	/* The data counts as a chunk */
+	alloc_stats.data_allocs += 1;
+	alloc_stats.alloc_ops += 1;
+	alloc_stats.active_allocs += 1;
+
+	data->tid.idx = idx;
+
+	bpf_spin_unlock(&alloc_lock);
+
+	return data;
+}
+
+/*
+ * Task BPF map entry recording the task's assigned ID and pointing to the data
+ * area allocated in arena.
+ */
+struct scx_task_map_val {
+	union sdt_id		tid;
+	__u64			tptr;
+	struct sdt_data __arena	*data;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, struct scx_task_map_val);
+} scx_task_map SEC(".maps");
+
+static struct scx_allocator scx_task_allocator;
+
+__hidden
+void __arena *scx_task_alloc(struct task_struct *p)
+{
+	struct sdt_data __arena *data = NULL;
+	struct scx_task_map_val *mval;
+
+	mval = bpf_task_storage_get(&scx_task_map, p, 0,
+				    BPF_LOCAL_STORAGE_GET_F_CREATE);
+	if (!mval)
+		return NULL;
+
+	data = scx_alloc(&scx_task_allocator);
+	if (unlikely(!data))
+		return NULL;
+
+	mval->tid = data->tid;
+	mval->tptr = (__u64) p;
+	mval->data = data;
+
+	return (void __arena *)data->payload;
+}
+
+__hidden
+int scx_task_init(__u64 data_size)
+{
+	return scx_alloc_init(&scx_task_allocator, data_size);
+}
+
+__hidden
+void __arena *scx_task_data(struct task_struct *p)
+{
+	struct sdt_data __arena *data;
+	struct scx_task_map_val *mval;
+
+	scx_arena_subprog_init();
+
+	mval = bpf_task_storage_get(&scx_task_map, p, 0, 0);
+	if (!mval)
+		return NULL;
+
+	data = mval->data;
+
+	return (void __arena *)data->payload;
+}
+
+__hidden
+void scx_task_free(struct task_struct *p)
+{
+	struct scx_task_map_val *mval;
+
+	scx_arena_subprog_init();
+
+	mval = bpf_task_storage_get(&scx_task_map, p, 0, 0);
+	if (!mval)
+		return;
+
+	bpf_spin_lock(&alloc_lock);
+	scx_alloc_free_idx(&scx_task_allocator, mval->tid.idx);
+	bpf_spin_unlock(&alloc_lock);
+
+	bpf_task_storage_delete(&scx_task_map, p);
+}
+
+static inline void
+scx_stat_global_update(struct scx_stats __arena *stats)
+{
+	cast_kern(stats);
+	__sync_fetch_and_add(&stat_enqueue, stats->enqueue);
+	__sync_fetch_and_add(&stat_init, stats->init);
+	__sync_fetch_and_add(&stat_exit, stats->exit);
+	__sync_fetch_and_add(&stat_select_idle_cpu, stats->select_idle_cpu);
+	__sync_fetch_and_add(&stat_select_busy_cpu, stats->select_busy_cpu);
+}
+
+s32 BPF_STRUCT_OPS(sdt_select_cpu, struct task_struct *p, s32 prev_cpu, u64 wake_flags)
+{
+	struct scx_stats __arena *stats;
+	bool is_idle = false;
+	s32 cpu;
+
+	stats = scx_task_data(p);
+	if (!stats) {
+		scx_bpf_error("%s: no stats for pid %d", __func__, p->pid);
+		return 0;
+	}
+
+	cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, &is_idle);
+	if (is_idle) {
+		stat_inc_select_idle_cpu(stats);
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
+	} else {
+		stat_inc_select_busy_cpu(stats);
+	}
+
+	return cpu;
+}
+
+void BPF_STRUCT_OPS(sdt_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	struct scx_stats __arena *stats;
+
+	stats = scx_task_data(p);
+	if (!stats) {
+		scx_bpf_error("%s: no stats for pid %d", __func__, p->pid);
+		return;
+	}
+
+	stat_inc_enqueue(stats);
+
+	scx_bpf_dsq_insert(p, SHARED_DSQ, SCX_SLICE_DFL, enq_flags);
+}
+
+void BPF_STRUCT_OPS(sdt_dispatch, s32 cpu, struct task_struct *prev)
+{
+	scx_bpf_dsq_move_to_local(SHARED_DSQ);
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(sdt_init_task, struct task_struct *p,
+			     struct scx_init_task_args *args)
+{
+	struct scx_stats __arena *stats;
+
+	stats = scx_task_alloc(p);
+	if (!stats) {
+		scx_bpf_error("arena allocator out of memory");
+		return -ENOMEM;
+	}
+
+	stats->pid = p->pid;
+
+	stat_inc_init(stats);
+
+	return 0;
+}
+
+void BPF_STRUCT_OPS(sdt_exit_task, struct task_struct *p,
+			      struct scx_exit_task_args *args)
+{
+	struct scx_stats __arena *stats;
+
+	stats = scx_task_data(p);
+	if (!stats) {
+		scx_bpf_error("%s: no stats for pid %d", __func__, p->pid);
+		return;
+	}
+
+	stat_inc_exit(stats);
+	scx_stat_global_update(stats);
+
+	scx_task_free(p);
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(sdt_init)
+{
+	int ret;
+
+	ret = scx_task_init(sizeof(struct scx_stats));
+	if (ret < 0) {
+		scx_bpf_error("%s: failed with %d", __func__, ret);
+		return ret;
+	}
+
+	return scx_bpf_create_dsq(SHARED_DSQ, -1);
+}
+
+void BPF_STRUCT_OPS(sdt_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SCX_OPS_DEFINE(sdt_ops,
+	       .select_cpu		= (void *)sdt_select_cpu,
+	       .enqueue			= (void *)sdt_enqueue,
+	       .dispatch		= (void *)sdt_dispatch,
+	       .init_task		= (void *)sdt_init_task,
+	       .exit_task		= (void *)sdt_exit_task,
+	       .init			= (void *)sdt_init,
+	       .exit			= (void *)sdt_exit,
+	       .name			= "sdt");

tools/sched_ext/scx_sdt.c

@@ -0,0 +1,101 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 Emil Tsalapatis <etsal@meta.com>
+ * Copyright (c) 2024 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
+#include <stdio.h>
+#include <unistd.h>
+#include <signal.h>
+#include <libgen.h>
+#include <bpf/bpf.h>
+#include <scx/common.h>
+
+#include "scx_sdt.h"
+#include "scx_sdt.bpf.skel.h"
+
+const char help_fmt[] =
+"A simple arena-based sched_ext scheduler.\n"
+"\n"
+"Modified version of scx_simple that demonstrates arena-based data structures.\n"
+"\n"
+"Usage: %s [-f] [-v]\n"
+"\n"
+"  -v            Print libbpf debug messages\n"
+"  -h            Display this help and exit\n";
+
+static bool verbose;
+static volatile int exit_req;
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	if (level == LIBBPF_DEBUG && !verbose)
+		return 0;
+	return vfprintf(stderr, format, args);
+}
+
+static void sigint_handler(int sig)
+{
+	exit_req = 1;
+}
+
+int main(int argc, char **argv)
+{
+	struct scx_sdt *skel;
+	struct bpf_link *link;
+	__u32 opt;
+	__u64 ecode;
+
+	libbpf_set_print(libbpf_print_fn);
+	signal(SIGINT, sigint_handler);
+	signal(SIGTERM, sigint_handler);
+restart:
+	skel = SCX_OPS_OPEN(sdt_ops, scx_sdt);
+
+	while ((opt = getopt(argc, argv, "fvh")) != -1) {
+		switch (opt) {
+		case 'v':
+			verbose = true;
+			break;
+		default:
+			fprintf(stderr, help_fmt, basename(argv[0]));
+			return opt != 'h';
+		}
+	}
+
+	SCX_OPS_LOAD(skel, sdt_ops, scx_sdt, uei);
+	link = SCX_OPS_ATTACH(skel, sdt_ops, scx_sdt);
+
+	while (!exit_req && !UEI_EXITED(skel, uei)) {
+		printf("====SCHEDULING STATS====\n");
+		printf("enqueues=%llu\t", skel->bss->stat_enqueue);
+		printf("inits=%llu\t", skel->bss->stat_init);
+		printf("exits=%llu\t", skel->bss->stat_exit);
+		printf("\n");
+
+		printf("select_idle_cpu=%llu\t", skel->bss->stat_select_idle_cpu);
+		printf("select_busy_cpu=%llu\t", skel->bss->stat_select_busy_cpu);
+		printf("\n");
+
+		printf("====ALLOCATION STATS====\n");
+		printf("chunk allocs=%llu\t", skel->bss->alloc_stats.chunk_allocs);
+		printf("data_allocs=%llu\n", skel->bss->alloc_stats.data_allocs);
+		printf("alloc_ops=%llu\t", skel->bss->alloc_stats.alloc_ops);
+		printf("free_ops=%llu\t", skel->bss->alloc_stats.free_ops);
+		printf("active_allocs=%llu\t", skel->bss->alloc_stats.active_allocs);
+		printf("arena_pages_used=%llu\t", skel->bss->alloc_stats.arena_pages_used);
+		printf("\n\n");
+
+		fflush(stdout);
+		sleep(1);
+	}
+
+	bpf_link__destroy(link);
+	ecode = UEI_REPORT(skel, uei);
+	scx_sdt__destroy(skel);
+
+	if (UEI_ECODE_RESTART(ecode))
+		goto restart;
+	return 0;
+}

tools/sched_ext/scx_sdt.h

@@ -0,0 +1,113 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0
+ * Copyright (c) 2025 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2025 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2025 Emil Tsalapatis <etsal@meta.com>
+ */
+#pragma once
+
+#ifndef __BPF__
+#define __arena
+#endif /* __BPF__ */
+
+struct scx_alloc_stats {
+	__u64		chunk_allocs;
+	__u64		data_allocs;
+	__u64		alloc_ops;
+	__u64		free_ops;
+	__u64		active_allocs;
+	__u64		arena_pages_used;
+};
+
+struct sdt_pool {
+	void __arena	*slab;
+	__u64		elem_size;
+	__u64		max_elems;
+	__u64		idx;
+};
+
+#ifndef div_round_up
+#define div_round_up(a, b) (((a) + (b) - 1) / (b))
+#endif
+
+#ifndef round_up
+#define round_up(a, b) (div_round_up((a), (b)) * (b))
+#endif
+
+typedef struct sdt_desc __arena sdt_desc_t;
+
+enum sdt_consts {
+	SDT_TASK_ENTS_PER_PAGE_SHIFT	= 9,
+	SDT_TASK_LEVELS			= 3,
+	SDT_TASK_ENTS_PER_CHUNK		= 1 << SDT_TASK_ENTS_PER_PAGE_SHIFT,
+	SDT_TASK_CHUNK_BITMAP_U64S	= div_round_up(SDT_TASK_ENTS_PER_CHUNK, 64),
+	SDT_TASK_MIN_ELEM_PER_ALLOC 	= 8,
+};
+
+union sdt_id {
+	__s64				val;
+	struct {
+		__s32			idx;	/* index in the radix tree */
+		__s32			genn;	/* ++'d on recycle so that it forms unique'ish 64bit ID */
+	};
+};
+
+struct sdt_chunk;
+
+/*
+ * Each index page is described by the following descriptor which carries the
+ * bitmap. This way the actual index can host power-of-two numbers of entries
+ * which makes indexing cheaper.
+ */
+struct sdt_desc {
+	__u64				allocated[SDT_TASK_CHUNK_BITMAP_U64S];
+	__u64				nr_free;
+	struct sdt_chunk __arena	*chunk;
+};
+
+/*
+ * Leaf node containing per-task data.
+ */
+struct sdt_data {
+	union sdt_id			tid;
+	__u64				payload[];
+};
+
+/*
+ * Intermediate node pointing to another intermediate node or leaf node.
+ */
+struct sdt_chunk {
+	union {
+		sdt_desc_t * descs[SDT_TASK_ENTS_PER_CHUNK];
+		struct sdt_data __arena *data[SDT_TASK_ENTS_PER_CHUNK];
+	};
+};
+
+struct scx_allocator {
+	struct sdt_pool	pool;
+	sdt_desc_t	*root;
+};
+
+struct scx_stats {
+	int	seq;
+	pid_t	pid;
+	__u64	enqueue;
+	__u64	exit;
+	__u64	init;
+	__u64	select_busy_cpu;
+	__u64	select_idle_cpu;
+};
+
+#ifdef __BPF__
+
+void __arena *scx_task_data(struct task_struct *p);
+int scx_task_init(__u64 data_size);
+void __arena *scx_task_alloc(struct task_struct *p);
+void scx_task_free(struct task_struct *p);
+void scx_arena_subprog_init(void);
+
+int scx_alloc_init(struct scx_allocator *alloc, __u64 data_size);
+u64 scx_alloc_internal(struct scx_allocator *alloc);
+int scx_alloc_free_idx(struct scx_allocator *alloc, __u64 idx);
+
+#endif /* __BPF__ */